1. Technical Field of the Invention
The present invention relates to the field winding type of synchronous rotary electric machine, and more particularly, to the improvement of field winding types of synchronous rotary electric machines which perform power supply by passing field current to a field winding (also referred to as “short-circuit winding”) from armature winding(s) in a non-contact manner using electromagnetic induction.
2. Related Art
Various types of rotary electric machines are known. As one type of such rotary electric machines, synchronous rotary electric machines are known. Such a synchronous rotary electric machine serves as a rotary electric machine by having its rotor rotated in synchronization with a rotating magnetic field formed by armature current, i.e. alternating current which flows through stator coils. Synchronous rotary electric machines are generally known to possess relatively high efficiency.
Rotors known to be used for such synchronous rotary electric machines include magnet type rotors, field winding type rotors, reluctance type rotors and rotors of the type having mixed properties of the foregoing rotors. The field winding type of synchronous rotary electric machines require neither to install expensive permanent magnets on the rotor cores, nor to consider resistance to centrifugal force of magnets. In addition, the field winding type of synchronous rotary electric machines can easily control torque or generated (induced) voltage by controlling field magnetic flux. Accordingly, the field winding type of synchronous rotary electric machines can exert good practicality when they are used as speed-variable rotary electric machines that generate motive power for vehicles to travel. However, the field winding type of synchronous rotary electric machines have suffered from an issue that they require to install and maintain a brush and slip-ring mechanism in order to supply field current to the field winding.
To address the issue mentioned above, Japanese Patent Laid-Open Publication No. 7-095790, for example, suggests a field winding type of synchronous rotary electric machine which is configured to effect PWM (pulse width modulation) control to armature current to the supply rotor-exciting current to the armature winding, the rotor-exciting current having frequency different from that of the rotor-synchronizing current. As a result, the alternating current induced to the field winding is rectified to serve as field current.
However, in the field current supply method of the type disclosed in this literature, fundamental components of armature current (hereinafter also referred to as “synchronizing current”) that synchronizes with the rotation of the rotor are superposed with the rotor exciting current for generating field current. For this reason, the efficiency of using field current corresponding to the amplitude of the fundamental components of the armature current becomes less relative to the amplitude of the DC power voltage. This issue of less efficiency has caused another issue of large torque ripple. Because of these issues, the field current supply method disclosed in the above literature has been prevented from being put into practical use.
To address for the issues mentioned above, Japanese Patent Laid-Open Publication No. 2007-185082 discloses a field winding type of synchronous rotary electric machine using a field current supply method, in which pulsed rotor-exciting current is superposed on the fundamental components of armature current forming the rotating field (hereinafter referred to as “pulsed rotor-exciting current superposing method”).
In the pulsed rotor-exciting current superposing method, pulsed rotor-exciting current having a cycle shorter than that of synchronizing current is superposed on the synchronizing current of one phase, for one or more number of times in one cycle of the synchronizing current. The advantage of superposing such pulsed rotor-exciting current is that the pulsed rotor-exciting current can be supplied during a favorable phase period of the synchronizing current of each phase. For example, in the case where the synchronizing current of a certain phase has a large amplitude in the positive direction, the reduction in the efficiency of using the DC power supply can be effectively avoided by passing pulsed rotor-exciting current in the negative direction.
As explained above, the pulsed rotor-exciting current superposing method can easily effect control by using PWM (pulse-width modulation) to control the voltage when supplying rotor-exciting current to three-phase PWM voltage for generating torque. In addition, this method can supply pulsed rotor-exciting current in the direction which is the reverse of the direction for supplying synchronizing current. Accordingly, comparing with a method using the rotor-exciting current having a continuous wave, the pulsed rotor-exciting current superposing method has an advantage of enhancing the efficiency of using DC power supply voltage. For example, in the case where the synchronizing current of a certain phase has large amplitude in the positive direction, the reduction in the efficiency of using DC power supply can be well avoided by supplying the pulsed rotor-exciting current in the negative direction.
However, supplying such pulsed rotor-exciting current has raised an issue that adjusting the required magnetic flux is difficult.
For example, when the duration of time for supplying the pulsed rotor-exciting current is increased, the field current passing through the rotor coils can be increased accordingly. However, the increased duration of time has caused larger strain in the waveform of phase current.
In particular, a speed-variable field winding type of synchronous rotary electric machine, when installed in a hybrid car or electric car, is required to frequently change the rate of rotations (rotation speed). In this case, the duration of time corresponding to one cycle of the synchronizing current in each phase is shortened in inverse proportion to the increase in the rate of rotations. For this reason, in a high-speed rotation region, the duration of time for supplying the pulsed rotor-exciting current having a constant time interval is relatively increased in one cycle of the synchronizing current of each phase. In response to the increase in the duration of time, the field current passing through the rotor coils is unavoidably increased, to also increase torque strain and torque ripple.
On the other hand, in a low-speed rotation region, the duration of time for supplying the pulsed rotor-exciting current having a constant time interval is relatively decreased in one cycle of the synchronizing current of each phase. Thus, in response to the decrease in the duration of time, the field current passing through the rotor coils is unavoidably decreased.